Calculates the size of a value. This is the default recursion
metric used for ACL2 termination proofs.

The size of a cons-pair is one more than the sum of the sizes of its
car and cdr. The size of an integer is its absolute value,
the size of a rational number is the sum of the sizes of its numerator and
denominator, and the size of a complex number is one more than the sum of the
sizes of its real and imaginary parts. The size of a string is its length. The
size of all other values (characters and symbols) is 0.